1. Field of the Invention
The invention belongs to the field of pressure vessel and safety engineering, in particular to a calibration method for the brittle fracture assessment parameters for materials, which is a calibration method for the brittle fracture assessment parameters for pressure vessel materials based on the Beremin cleavage fracture model.
2. Related Art
Nuclear power has become an important part of the world's energy structure. Currently, there are 11 reactors in use in our country. In accordance with China's medium and long-term development plan of “Developing Nuclear Power Actively”, there will be more than 40 new reactors which are the third generation million-kilowatt advanced pressurized water reactor nuclear power plants as the representative of AP1000 in 15 years. Our country will develop the nuclear power most rapidly in the world. As the key component of the nuclear power plant, reactor pressure vessel is made of ferritic steel, which demonstrates a strong transition phenomenon from ductile to brittle. During service, the steel in the reactor pressure vessel beltline region is subject to neutron irradiation, which results in an upward shift in the transition temperature. In other words, the fracture toughness of the steels decreases within the specified operating temperature. It is very necessary to ensure the structural integrity assessment of the pressure vessels, especially the reactor pressure vessels, under the different possible conditions in the design, operation and maintenance stages to prevent any possible brittle fracture. The fracture toughness of materials (including the base, weld and heat-affected zone materials) is essential to the structural integrity assessment.
Local approach to cleavage fracture is a primary method for predicting brittle failure probability for ferritic pressure vessel steel. Among them, the most widely applied model is the Beremin model which has been included in the famous R6 Procedure “Assessment of the Integrity of Structures Containing Defects”. The Beremin model was originally proposed by the research group F.M Beremin for studying cleavage fracture of pressure vessel steels. The Beremin model is very applicable to the analysis of the effect of constraint on cleavage fracture toughness and to the prediction of cleavage fracture of steels subjected to complex loading conditions such as multi-axial loading and high strain rate loading.
The Beremin model uses only two parameters, the Weibull slope m and Weibull scale parameter σu, to describe the complex cleavage fracture events. Therefore, the applicability of the Beremin model to predict cleavage fracture in structures relies heavily on the model's parameters. The calibration method for Beremin model's parameters is a key technology for the brittle fracture assessment procedure for pressure vessel materials.
Several calibration methods have been reported in the literatures. For example, in 1992, Minami et al published “Estimation procedure for the Weibull stress parameters used in the local approach” in the journal “International Journal of Fracture”, in which a calibration method using a maximum likelihood analysis of a single set of fracture toughness values for high constraint specimens was proposed; in 1998, a paper entitled “Calibration of Weibull stress parameters using fracture toughness data” published by Gao et al in the journal “Engineering fracture mechanics” first describes a calibration method (GRD method) based on the analysis of two sets of fracture toughness values exhibiting different constraint levels at fracture; in 2000, Ruggieri et al's (RGD) paper “Transferability of elastic-plastic fracture toughness using the Weibull stress approach: significance of parameter calibration” published in the journal “Engineering Fracture Mechanics” simplifies the GRD method.
However, the existing methods require a lot of complex calculations and sometimes a specialized computer program. In particular, the calibration method proposed by Minami et al must need a specialized computer program that employs an iterative process to obtain m and σu. When the RGD method is utilized, the maximum principal stress and volume of every element first need to be extracted from the fracture process region of each model at different loading levels, assume several trial values of m and do a lot of calculations to build the σw vs. KJ relationships for each type of specimens using the exported data, and finally construct the toughness scaling diagrams between the two different specimens based on equal σw values. The method is computationally expensive.
In addition, the calibration method proposed by Minami et al is based on the analysis of a single set of fracture toughness data for high constraint specimen, which results in large uncertainty in the calibrated Beremin model's parameters and poor transferability of the calibrated parameters across structures of different constraints. The RGD calibration and the GRD calibration method determine parameters (m, σu) using two sets of fracture toughness data obtained for high constraint and low constraint specimens, but can't tune (m, σu) by using more than two types of specimens simultaneously. Moreover, when there are equivalent solutions for the model's parameters, the GRD calibration method and the RGD calibration method only give the most accurate solution for the parameters (m, σu), but neglect the other solutions.